add meetup exercise, part of 48 in 24

config.json

@@ -862,6 +862,14 @@
         "practices": [],
         "prerequisites": [],
         "difficulty": 5
+      },
+      {
+        "slug": "meetup",
+        "name": "Meetup",
+        "uuid": "80000759-8ca8-4ff8-a88f-34c4244ac6c5",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 5
       }
     ]
   },

exercises/practice/meetup/.docs/instructions.md

@@ -0,0 +1,34 @@
+# Instructions
+
+Your task is to find the exact date of a meetup, given a month, year, weekday and week.
+
+There are five week values to consider: `first`, `second`, `third`, `fourth`, `last`, `teenth`.
+
+For example, you might be asked to find the date for the meetup on the first Monday in January 2018 (January 1, 2018).
+
+Similarly, you might be asked to find:
+
+- the third Tuesday of August 2019 (August 20, 2019)
+- the teenth Wednesday of May 2020 (May 13, 2020)
+- the fourth Sunday of July 2021 (July 25, 2021)
+- the last Thursday of November 2022 (November 24, 2022)
+- the teenth Saturday of August 1953 (August 15, 1953)
+
+## Teenth
+
+The teenth week refers to the seven days in a month that end in '-teenth' (13th, 14th, 15th, 16th, 17th, 18th and 19th).
+
+If asked to find the teenth Saturday of August, 1953, we check its calendar:
+
+```plaintext
+    August 1953
+Su Mo Tu We Th Fr Sa
+                   1
+ 2  3  4  5  6  7  8
+ 9 10 11 12 13 14 15
+16 17 18 19 20 21 22
+23 24 25 26 27 28 29
+30 31
+```
+
+From this we find that the teenth Saturday is August 15, 1953.

exercises/practice/meetup/.docs/introduction.md

@@ -0,0 +1,29 @@
+# Introduction
+
+Every month, your partner meets up with their best friend.
+Both of them have very busy schedules, making it challenging to find a suitable date!
+Given your own busy schedule, your partner always double-checks potential meetup dates with you:
+
+- "Can I meet up on the first Friday of next month?"
+- "What about the third Wednesday?"
+- "Maybe the last Sunday?"
+
+In this month's call, your partner asked you this question:
+
+- "I'd like to meet up on the teenth Thursday; is that okay?"
+
+Confused, you ask what a "teenth" day is.
+Your partner explains that a teenth day, a concept they made up, refers to the days in a month that end in '-teenth':
+
+- 13th (thirteenth)
+- 14th (fourteenth)
+- 15th (fifteenth)
+- 16th (sixteenth)
+- 17th (seventeenth)
+- 18th (eighteenth)
+- 19th (nineteenth)
+
+As there are also seven weekdays, it is guaranteed that each day of the week has _exactly one_ teenth day each month.
+
+Now that you understand the concept of a teenth day, you check your calendar.
+You don't have anything planned on the teenth Thursday, so you happily confirm the date with your partner.

exercises/practice/meetup/.meta/config.json

@@ -0,0 +1,18 @@
+{
+  "authors": [
+    "kmarker1101"
+  ],
+  "files": {
+    "solution": [
+      "meetup.el"
+    ],
+    "test": [
+      "meetup-test.el"
+    ],
+    "example": [
+      ".meta/example.el"
+    ]
+  },
+  "blurb": "Calculate the date of meetups.",
+  "source": "Jeremy Hinegardner mentioned a Boulder meetup that happens on the Wednesteenth of every month"
+}

exercises/practice/meetup/.meta/example.el

@@ -0,0 +1,61 @@
+;;; meetup.el --- Meetup (exercism)  -*- lexical-binding: t; -*-
+
+;;; Commentary:
+
+;;; Code:
+
+(require 'calendar)
+
+(defun last-day-of (month year)
+  (calendar-last-day-of-month month year))
+
+(defun find-dow-near-date (target-dow start-day month year direction)
+  (let* ((direction-factor (if (eq direction :before) -1 1))
+         (current-day start-day)
+         (found-day nil))
+    (while (and (not found-day)
+                (<= 1 current-day (last-day-of month year)))
+      (let ((current-dow (calendar-day-of-week (list month current-day year))))
+        (when (= current-dow target-dow)
+          (setq found-day current-day)))
+      (setq current-day (+ current-day direction-factor)))
+    (list year month found-day)))
+
+(defun meetup (month year dow schedule)
+  (let* ((target-dow (cdr (assoc dow +days-of-the-week+)))
+         (first-day-of-month (calendar-day-of-week (list month 1 year)))
+         (schedule-offset (cdr (assoc schedule +schedules+)))
+         (first-target-dow (+ 1 (- target-dow first-day-of-month)))
+         (first-occurrence (if (>= first-target-dow 1)
+                               first-target-dow
+                             (+ first-target-dow 7)))
+         (start-day (cond
+                     ((eq schedule :last)
+                      (last-day-of month year))
+                     ((eq schedule :teenth)
+                      13)
+                     (t
+                      (+ first-occurrence (* (/ (1- schedule-offset) 7) 7))))))
+    (if (eq schedule :last)
+        (find-dow-near-date target-dow start-day month year :before)
+      (find-dow-near-date target-dow start-day month year :after))))
+
+(defconst +days-of-the-week+
+  '((:sunday . 0)
+    (:monday . 1)
+    (:tuesday . 2)
+    (:wednesday . 3)
+    (:thursday . 4)
+    (:friday . 5)
+    (:saturday . 6)))
+
+(defconst +schedules+
+  '((:first . 1)
+    (:second . 8)
+    (:third . 15)
+    (:teenth . 13)
+    (:fourth . 22)
+    (:last . -7)))
+
+(provide 'meetup)
+;;; meetup.el ends here